Apparatus for severing paper sheets or the like

ABSTRACT

A severing apparatus for stepwise advancing paper webs or the like has a mobile knife which is reciprocable into and out of the gap between two stationary counterknives. The counterknives are pivotable in their support and are biased against the mobile knife by means of hoses consisting of elastomeric material. Two projections of the mobile knife extend into the gap between and are in permanent contact with the counterknives. The mobile knife has two V-shaped cutting edges which cooperate with straight cutting edges of the counterknives to sever the web from the marginal portions toward the center and to separate from the web a narrow strip which descends into the gap between the counterknives. One of the counterknives is detachable to allow for conversion of the severing apparatus into a means for making simple cuts without removal of material from the web. The bias of at least one of the hoses is adjustable and the plane of at least one counterknife is inclined relative to the plane of the mobile knife to permit for gravitational descent of removed strips through the gap between the counterknives.

United States Patent [191 Nagel et al.

[ Mar. 26, 1974 APPARATUS FOR SEVERING PAPER SHEETS OR THE LIKE [75] Inventors: Erich Nagel, Anzing; Guenter Engelage, Unterhaching; Lothar Himmelsbach, Munich; Gerhard Frankiewicz, Sauerlach, all of Germany [73] Assignee: AGFA-Gevaert Aktiengesellschaft,

Leverkusen, Germany [22] Filed: July 25, 1972 [21] Appl. N0.: 275,076

[30] Foreign Application Priority Data July 31, 1971 Germany 2138420 [52] US. Cl 83/583, 83/636, 83/620, 83/694 [51] lnt. Cl B26d 5/08 [58] Field of Search 83/620, 583, 636, 694

[5 6] References Cited UNITED STATES PATENTS 3,218,918 11/1965 Brindley et a1. 83/583 442,823 12/1890 Palm 83/636 629,532 7/1899 Thomas 83/636 X 2,626,664 l/l953 Regele 83/694 X FOREIGN PATENTS OR APPLICATIONS 814,591 3/1937 France 83/583 Primary Examiner-Donald R. Schran Attorney, Agent, or Firm-Michael S. Striker [5 7] ABSTRACT A severing apparatus for stepwise advancing paper webs or the like has a mobile knife which is reciprocable into and out of the gap between two stationary counterknives. The counterknives are pivotable in their support and are biased against the mobile knife by means of hoses consisting of elastomeric material. Two projections of the mobile knife extend into the gap between and are in permanent contact with the counterknives. The mobile knife has two V-shaped cutting edges which cooperate with straight cutting edges of the counterknives to sever the web from the marginal portions toward the center and to separate from the web a narrow strip which descends into the gap between the counterknives. One of the counterknives is detachable to allow for conversion of the severing apparatus into a means for making simple cuts without removal of material from the web. The bias of at least one of the hoses is adjustable and the plane of at least one counterknife is inclined relative to the plane of the mobile knife to permit for gravitational descent of removed strips through the gap between the counterknives.

16 Claims, 3 Drawing Figures PAIENTED R2 1974 SHEET 2 0F 3 APPARATUS FOR SEVERING PAPER SHEETS OR THE LIKE BACKGROUND OF THE INVENTION The present invention relates to apparatus for severing sheet-like materials, such as elongated paper webs having layers of exposed and developed photosensitive material.

It is already known to sever paper webs in an apparatus wherein the web is fed stepwise and a mobile knife is reciprocable relative to a stationary counterknife. In order to insure the making of clean cuts, the knives must be machined and sharpened with a high degree of precision. As a rule, the knives are relatively thin, especially the mobile knife, and at least the mobile knife is normally hardened so as to be capable of standing relatively long periods of use. The hardening invariably results in at least some distortion or deformation of the mobile knife so that the latter must be subjected to a secondary treatment, such as grinding with magnetic retention or mounting during grinding. It was found that even such secondary treatment does not reliably eliminate or compensate for deformation which develops during hardening, mainly because the magnetic mounting during grinding involves at least some elastic deformation of the knife. Such elastic deformation disappears as soon as the application of magnetic force is terminated with the result that the distortion which developed during hardening is merely diminished or reappears in its entirety. This affects the quality of cuts which is particularly undesirable when the knife is used in an apparatus for subdividing an elongated paper web with reproductions of images of film frames into a succession of discrete prints.

SUMMARY OF THE INVENTION An object of the invention is to provide a novel and improved severing apparatus wherein the knives can make satisfactory cuts even if they are not machined and finished with a high degree of precision.

Another object of the invention is to provide a novel and improved mounting for the mobile knife and one or more counterknives in a severing apparatus for webs consisting of paper or the like.

A further object of the invention is to provide a severing apparatus wherein the mobile knife is capable of making satisfactory cuts even ifit exhibits the tendency to deform as a result of a hardening or analogous treatment.

An additional object of the invention is to provide a severing apparatus which can be rapidly and conveniently converted from the making of simple cuts to the removal of strip-shaped portions from paper webs or vice versa.

Still another object of the invention is to provide a severing apparatus wherein the wear upon the knives does not unduly affect the quality of cuts and which is capable of automatically sharpening its knives when in use.

An additional object of the invention is to provide a severing apparatus wherein the material which is being removed from paper webs or the like can be automatically evacuated to prevent its accumulation around or between the knives and wherein each of the knives can be inspected, exchanged or replaced with little loss in time.

The improved apparatus is particularly suited for severing of sheet material in the form of paper webs having layers of exposed and developed photosensitive material. The apparatus comprises a support, counterknife means including at least one stationary knife mounted in the support, a mobile knife movable with reference to the stationary knife forwardly and rearwardly in a predetermined direction and having a portion (e.g., in the form of two spaced-apart projections) permanently abutting against the stationary knife and at least one cutting edge which is inclined with reference to the direction of movement of the mobile knife, drive means for moving the mobile knife whereby the latter cooperates with the stationary knife to sever during its forward movement a sheet material which is placed between the knives, and biasing means for yieldably urging one of the knives (e.g., the stationary knife) toward the other knife. Such biasing means may comprise one or more elastomeric hoses or the like which can be interposed between the stationary knife and its support to bias that portion of the stationary knife which is provided with the cutting edge into surface-to-surface abutment with the adjacent side of the mobile knife.

The novel features which are considered as characteristic of the invention are set forth in particular in the appended claims. The improved severing apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain specific embodiments with reference to the accompanying drawing.

BRIEF DESCRIPTION OF THE DRAWING FIG. 1 is a diagrammatic front elevational view of a severing apparatus which embodies the invention;

FIG. 2 is a side elevational view of the severing apparatus as seen from the left-hand side of FIG. 1; and

FIG. 3 is an enlarged transverse vertical sectional view as seen in the direction of arrows from the line III- -III of FIG. 1.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to FIGS. 1 and 3, the severing apparatus comprises a counterknife support including a first portion 11 and a removable second portion 12. The portion 11 supports a first stationary knife 8 having at its upper end a straight horizontal cutting edge 80, and the portion 12 supports a second stationary knife 9 having at its upper end a straight horizontal cutting edge 9a located at a level slightly below the cutting edge 8a. The severing apparatus further comprises a mobile knife 1 which is a relatively thin plate having two inverted V-shaped cutting edges la and being guided in a gap or passage 4 between two guide plates 24, 25 with minimal clearance or with no clearance at all so that it can reciprocate by moving up and down (arrow 3) in a vertical plane coinciding with the plane of the gap 30 between the stationary knives 8 and 9. The cutting edges la of the mobile knife 1 have portions which are inclined relative to each other as well as relative to the direction of forward and rearward movement of the knife 1.

The means for feeding a sheet material 2 (e.g., a web of paper one side of which is coated with a layer of exposed and developed photosensive material so that the web forms a series of interconnected photographic prints) includes a pair of advancing rolls 20, 21 (see FIG. 2) which are driven at regular intervals to withdraw the sheet material from a bobbin 22 and to advance the material through distances corresponding to those between the neighboring frame lines on the photosensitive layer. The advancing rolls 20, 21 are at a standstill when the mobile knife 1 is caused to move forwardly (arrow 3) and to sever the material 2 across a frame line which overlies the gap 30. Due to the V- shaped configuration of its cutting edges 1a, the mobile knife 1 severs the material 2 from the marginal portions toward its center which is desirable in order to reduce the likelihood of application of flexing forces which could affect the quality of cuts.

The guide plates 24, 25 contribute to retention of the mobile knife 1 in a predetermined plane. Their mutual spacing can be adjusted by replacing the illustrated distancing sleeves 5 with longer or shorter distancing sleeves. The sleeves 5 are mounted on bolts 6 which mesh with nuts 7. FIG. 3 shows that the knife 1 is hollow ground between its V-shaped cutting edges la which enables such cutting edges to remove from the material 2 narrow strips (i.e., to form so-called stamping cuts) whereby the removed strips descend into the gap 30 between the stationary knives 8,9. When the knife 9 is removed, one of the cutting edges 1a cooperates with the cutting edge 8a of the knife 8 to form a simple cut without removal of material from the web 2.

In accordance with a feature of the invention, the mobile knife 1 comprises a portion (here shown as including two spaced apart projections or lugs lb, 10) which is in permanent engagement with the stationary knives 8 and 9. The projections lb, 1c extend into the gap 30 at the opposite sides of the path for the material 2; such path extends between the knives 1 and 8,9 when the knife 1 is retracted to assume the position shown in FIG. 1.

The drive means for moving the mobile knife 1 in and counter to the direction indicated by the arrow 3 includes a yoke 10 which has an eye 10d (FIG. 2) for a pin (not shown) adapted to be moved up and down by the eccentric output element ofa suitable prime mover in order to move the knife 1 downwardly and thereupon back to the retracted position while the material 2 is at a standstill. The upper portion of the yoke 10 is provided with two projections 10a, 10b which extend into sockets 1d, 1e provided in the end portions of the mobile knife 1 so that the latter is compelled to share the up-and-down movements of the yoke 10 without changing its orientation relative to the stationary knives 8 and 9. The prime mover for moving the yoke 10 of the drive means up and down may comprise an electric motor whose shaft performs a complete revolution when the advancing rolls 20, 21 are at a standstill whereby the yoke 10 moves in and counter to the direction indicated by the arrow 3 to thus compel the knife 1 to remove from the material 2 a narrow strip which descends into the gap 30. The yoke 10 is sufficiently rugged to move the knife 1 up and down and to take up all such stresses which would tend to tilt the knife 1 relative to the knives 8 and 9.

Referring in detail to FIG. 3, it will be noted that the stationary knife 8 is provided with an elongated rounded lower edge portion 8b which rests on a platform of the support portion 11 and enables the knife 8 to pivot so as to move its straight cutting edge toward or away from the cutting edge 9a of the knife 9. The latter is also provided with an elongated rounded edge portion 9b which enables it to pivot relative to the removable support portion 12. The means for releasably securing the portion 12 to the portion 11 of the support for the knives 8, 9 includes two screws 18, 19 or analogous fastener means. The support portion 11 further carries a claw 14 having a pin 23 which extends into a recess or bore 8c of the knife 8 so as to hold the latter against movement in response to reciprocation of the mobile knife 1. The lower portion of the claw 14 has a vertical slot 14a for the stem of a screw 13 which meshes with the support portion 11. By loosening the screw 13, the operator can move the knife 8 up or down, for example, to compensate for wear on the cutting edge 8a and/or to change the distance between the levels of the cutting edges 8a and 9a. The pin 23 does not prevent a pivotal movement of the knife 8 about the line of contact between the rounded edge portion 8b and the support portion 11 but preferably insures that the knife 8 has limited freedom or no freedom of movement in or counter to the direction indicated by the arrow 3.

In accordance with another feature of the invention, the stationary knife 8 is biased against the adjacent side of the mobile knife 1 by a biasing means here shown as a hose 15 consisting of rubber or other suitable elastomeric material. The hose 15 is inserted into a recess 16a of a holder 16 which is mounted on the support portion 1 1 and is adjustable in and counter to the direction indicated by arrow 31. This enables an operator to select the bias of the hose 15 upon the knife 8. For example, the holder 16 can be provided with two or more elongated slots to receive the shanks of screws which mesh with the support portion 11. By loosening such screws, the operator can shift the holder 16 toward or away from the knife 9 to thereby change the bias of the hose 15. Since the hose 15 is not subjected to any appreciable wear, it can be made of ordinary rubber or the like and is still capable of standing long periods of use at normal temperatures without becoming brittle. In addition, the hose 15 performs a desirable sealing function by preventing the penetration of dust or other foreign matter into the space between the support portion 11 and the knife 8. Any foreign matter which would be allowed to penetrate between the parts 8 and 11 would be likely to prevent counterclockwise pivoting of the knife 8, as viewed in FIG. 3.

The manner in which the knife 9 is mounted in the detachable support portion 12 is similar to the mounting of the knife 8. The drawing shows a claw 14' having a pin 23' and a recess in the lower portion of the knife 9. The support portion 12 has a recess 12a for a second biasing hose 17 which urges the knife 9 in a counterclockwise directon, as viewed in FIG. 3, i.e., against the adjacent side of the mobile knife 1. The bias of the hose 17 need not be adjusted becausethe support portion 12 is preferably adjustable relative to the support portion 11. The conversion of the apparatus from removal of strips to the making of simple transverse cuts or vice versa can be completed with minimal losses in time, merely by removing the detachable support portion 12 with the knife 9 or by reattaching the portion 12 to the portion 11 by means of the fasteners 18, 19.

As mentioned before and as shown in FIG. 3, the cutting edge 90 of the knife 9 is preferably offset (located at a level below) relative to the cutting edge 8a of the knife 8. The extent of such offset can be in the range of between 1/6 and U of the thickness of the platelike main body portion of the mobile knife 1. The positioning of the cutting edge 90 at a level below the cutting edge 8a insures that, when the rolls 20, 21 advance the material 2 by a step so that the leading edge of the material 2 is moved beyond the cutting edge 8a (which is nearer to the rolls 20, 21 than the cutting edge 9a), the material 2 is not likely to touch the cuttinge edge 90 due to its tendency of curl or due to the weight of that portion which has been advanced beyond the knife 8. The aforementioned extent of offset between the cutting edges 8a, 9a has been found to be invariably sufflcient to prevent any contact between the material 2 and the knife 9 prior to downward or forward movement of the mobile knife 1. On the other hand, the difference between the levels of the cutting edges 8a and 90 should be small enough to insure that the knife 1 begins to cooperate with the knife 9 in order to cut into the material 2 before the cutting by means of the knives 1 and 8 is completed. This can be readily achieved by properly selecting the magnitude of the angle between the two mutually inclined portions of each cutting edge la on the mobile knife 1.

The operation is as follows:

The drive for the advancing rolls 20, 21 is started while the knife 1 dwells in its raised position whereby the rolls 20, 21 feed the sheet material 2 along a horizontal path which extends between the cutting edges la and the cutting edges 8a, 9a. The rolls 20, 21 are arrested when the material 2 is advanced by the length of a print. in the next step, the yoke 10 of the drive means for the knife 1 is caused to move in the direction indicated by arrow 3 whereby the projections 10a, 10b entrain the knife 1 and the latter cooperates with the stationary knives 8, 9 to remove from the material 2 a narrow strip which descends into the gap 30. The projections lb, 10 of the mobile knife I extend into the gap at the opposite sides of the path for the material 2 so that they are permanently engaged by the knife 9 and/or 8 due to the bias of the elastic hose 17 and/or 15. The hoses 15, 1-7 normally tend to flex the median portions of knives 8, 9 toward each other so that such median portions are moved apart by the main or central body portion of the knife 1 when the latter descends with the yoke 10. Thus, as the knife 1 moves downwardly, it finds its way into the gap 30 between the knives 8, 9 with resulting stressing of the hoses 15, 17 so that the cutting edges 1a slide along the respective cutting edges 8a, 9a without any clearance to sever the material 2 in two parallel planes, always from the marginal portions toward the center of the material. Since the mutually inclined portions of each cutting edge 1a are preferably mirror symmetrical to each other with reference to a central plane of the apparatus (see the line IIllll of HO. 1), the severing action takes place in two relatively short regions which travel at identical speeds from the marginal portions toward the center of the material 2 while the knife 1 is caused to move in the direction indicated by arrow 3.

When the left-hand cutting edge 1a of the knife 1 (as viewed in FlG. 3) reaches the marginal portions of the material 2, the material is clamped at two spaced-apart points and is thereupon severed as the points travel toward each other, i.e., toward the line llllll of FIG. 1. Since the inclination of both portions of each cutting edge 1a relative to the direction of movement of the knife 1 is invariably smaller than the angle which is needed to bring about a self-locking action, the descending knife 1 cannot cause any lengthwise displacement of the material. At the very most, the material 2 exhibits a slight tendency to arch without in any way affecting the accuracy of the severing operation. When the severing step is completed, the knife 1 is returned to its retracted or raised position and the advancing rolls 20, 21 are set in motion to feed the material 2 in a direction from the bobbin 22 toward and through the space between the cutting edges 1a and 8a, 9a. The freshly severed leading end of the material 2 descends into a tray or a like intercepting receptacle 33 shown in FIG. 2.

FIG. 3 further shows that the elastic hoses 15, 17 maintain the respective stationary knives 8, 9 in planes which are slightly inclined with reference to the plane of the mobile knife 1. The angle is normally between 1 and 6, preferably about 3. Such inclination of the planes of knives 8, 9 with reference to the plane of the knife 1 insures that the width of the gap 30 increases in a downward direction with the result that the strips which are removed by the knife 1 in cooperation with the knives 8, 9 are free to descend by gravity and cannot interfere with pivoting of the knives 8, 9 and/or with reciprocatory movements of the knife 1. Moreover, such inclination of the knives 8, 9 with reference to the knife 1 insures the formation of satisfactory cuts, even if the knife 1 is being moved at a very high speed and at frequent intervals, because such mounting reduces the wear on the knives and prevents the formation of shoulders on the knives 8, 9 adjacent to the gap 30.

The improved severing apparatus can be used for severing of a wide variety of sheet materials, such as paper webs, webs of photographic film, splices between united webs of photographic film and many other materials whose resistance to severing may be much higher than that of paper. Also, the improved severing apparatus is susceptible of many additional modifications without departing from the spirit of the invention. For example, the support means for the knives 8, 9 can be designed in such a way that the operator can remove the knife 8 or 9, especially if the apparatus is mounted for severing of material which is being fed in a direction from the left to the right, or vice versa, as viewed in FIG. 2. Also, the knife 1 may be mounted at a level below the knives 9 and/or 8, and the rolls 20, 21 may be mounted to pull (rather than push) the sheet mate rial through the space between the stationary knive or knives and the mobile knife.

An important advantage of our severing apparatus is that the mobile knife can be readily held against undue distortion even though it exhibits a tendency to deform. Therefore, this knife need not be machined with a very high degree of precision. Due to the fact that the projections lb, 1c of the knife 1 are in permanent engagement with the knives 8, 9 and that the elastic members 15, 17 bias the flexible knives 8, 9 against the descending knife 1, the clearance between the contacting portions of cutting edges 1a, 8a, 9a is zero. The flexing of the knives 8, 9 does not unduly stress the support portions 11, 12 and/or the guide members 5.

Without further analysis, the foregoing will so fully reveal the gist of the present invention that others can, by applying current knowledge, readily adapt it for various applications without omitting features which fairly constitute essential characteristics of the generic and specific aspects of our contribution to the art and, therefore, such adaptations should and are intended to be comprehended within the meaning and range of equivalence of the claims.

What is claimed as new and desired to be protected by Letters Patent is set forth in the appended claims.

We claim:

1. Apparatus for severing sheet material, particularly for severing paper webs having layers of exposed and developed photosensitive material, comprising a support; two counterknives mounted in said support; a mobile knife movable with reference to said counterknives forwardly and rearwardly in a predetermined direction, said counterknives defining a gap for reception of said knife during said forward movement of said knife and said knife having a portion abutting against said counterknives and at least one cutting edge which is inclined relative to said direction, said counterknives comprising cutting edges which are offset relative to each other so that, while said knife moves forwardly, it first engages the cutting edge of one counterknife and thereupon the cutting edge of the other counterknife; drive means for moving said knife whereby the knife cooperates with said counterknives to sever during the forward movement thereof a sheet material which is placed between said knife and said counterknives; and biasing means for yieldably urging said counterknives against said knife.

2. Apparatus as defined in claim 1, further comprising advancing means for intermittently moving sheet material along a predetermined path extending between said knife and counterknives and for maintaining the sheet material at a standstill during said forward movement of said mobile knife, said portion of said mobile knife being located outside of said path.

3. Apparatus as defined in claim 1, wherein said drive means comprises means for reciprocating said mobile knife and for holding said mobile knife against a change in orientation relative to said counterknives.

4. Apparatus as defined in claim 1, wherein said one cutting edge is a substantially V-shaped cutting edge arranged to sever the sheet material in directions from the ends toward the center of said cutting edge during forward movement of said mobile knife.

5. Apparatus as defined in claim 1, wherein said one cutting edge is a hollow ground cutting edge.

6. Apparatus as defined in claim 1, wherein said mobile knife comprises a substantially plate-like main body portion and further comprising guide means defining a passage for at least substantially clearance-free reciprocatory movement of said main body portion in said predetermined direction, said counterknives having rounded edge portions engaging with and being pivotable relative to said support, said biasing means being interposed between said support and said counterknives to pivot said counterknives toward engagement with said main body portion of said mobile knife.

7. Apparatus as defined in claim 1, further comprising adjustable stressing means for said biasing means.

8. Apparatus as defined in claim 1, wherein said mobile knife comprises two cutting edges one of which cooperates with the cutting edge of one and the other of which cooperates with the cutting edge of the other counterknives whereby said cutting edges remove from the sheet material a narrow strip during toward move ment of said mobile knife.

9. Apparatus as defined in claim 1, wherein the extent of said offset approximates between H6 and H10 of the thickness of said mobile knife.

10. Apparatus as defined in claim 1, wherein said mobile knife is located in a first plane and one of said counterknives is located in a second plane normally making with said first plane an angle of 1-6.

11. Apparatus as defined in claim 1, wherein one of said counterknives is provided with at least one socket and further comprising a projection mounted in said support and extending into said socket to hold said one counterknife against movement in said direction.

12. Apparatus as defined in claim 11, further comprising means for adjustably securing said projection to said support.

13. Apparatus as defined in claim 1, further comprising advancing means for moving sheet material stepwise along a predetermined path extending between said knife and said counterknives, said portion of said mobile knife comprising two spaced apart projections located at the opposite sides of said path and each abutting against said counterknives.

14. Apparatus as defined in claim 13, whereinsaid mobile knife further comprises two spaced apart recesses located at the opposite sides of said path and said drive means comprises projections extending into said recesses.

15. Apparatus as defined in claim 14, wherein said drive means comprises a reciprocable yoke which is provided with said projections.

16. Apparatus for severing sheet material, particularly for severing paper webs having layers of exposed and developed photosensitive material, comprising a support; two counterknives mounted in said support, said support comprising first and second portions which respectively support one and the other of said counterknives and means for separably securing one of said portions to the other portion so as to facilitate removal of the respective counterknife; a mobile knife movable with reference to said counterknives forwardly and rearwardly in a predetermined direction, said counterknives defining a gap for reception of said knife during said forward movement of said knife and said knife having a portion abutting against said counterknives and at least one cutting edge which is inclined relative to said direction; drive means for moving said knife whereby the latter cooperates with said counterknives to sever during the forward movement thereof a sheet material which is placed between said knife and said counterknives; and biasing means for yieldably urging said counterknives against said knife. 

1. Apparatus for severing sheet material, particularly for severing paper webs having layers of exposed and developed photosensitive material, comprising a support; two counterknives mounted in said support; a mobile knife movable with reference to said counterknives forwardly and rearwardly in a predetermined direction, said counterknives defining a gap for reception of said knife during said forward movement of said knife and said knife having a portion abutting against said counterknives and at least one cutting edge which is inclined relative to said direction, said counterknives comprising cutting edges which are offset relative to each other so that, while said knife moves forwardly, it first engages the cutting edge of one counterknife and thereupon the cutting edge of the other counterknife; drive meaNs for moving said knife whereby the knife cooperates with said counterknives to sever during the forward movement thereof a sheet material which is placed between said knife and said counterknives; and biasing means for yieldably urging said counterknives against said knife.
 2. Apparatus as defined in claim 1, further comprising advancing means for intermittently moving sheet material along a predetermined path extending between said knife and counterknives and for maintaining the sheet material at a standstill during said forward movement of said mobile knife, said portion of said mobile knife being located outside of said path.
 3. Apparatus as defined in claim 1, wherein said drive means comprises means for reciprocating said mobile knife and for holding said mobile knife against a change in orientation relative to said counterknives.
 4. Apparatus as defined in claim 1, wherein said one cutting edge is a substantially V-shaped cutting edge arranged to sever the sheet material in directions from the ends toward the center of said cutting edge during forward movement of said mobile knife.
 5. Apparatus as defined in claim 1, wherein said one cutting edge is a hollow ground cutting edge.
 6. Apparatus as defined in claim 1, wherein said mobile knife comprises a substantially plate-like main body portion and further comprising guide means defining a passage for at least substantially clearance-free reciprocatory movement of said main body portion in said predetermined direction, said counterknives having rounded edge portions engaging with and being pivotable relative to said support, said biasing means being interposed between said support and said counterknives to pivot said counterknives toward engagement with said main body portion of said mobile knife.
 7. Apparatus as defined in claim 1, further comprising adjustable stressing means for said biasing means.
 8. Apparatus as defined in claim 1, wherein said mobile knife comprises two cutting edges one of which cooperates with the cutting edge of one and the other of which cooperates with the cutting edge of the other counterknives whereby said cutting edges remove from the sheet material a narrow strip during toward movement of said mobile knife.
 9. Apparatus as defined in claim 1, wherein the extent of said offset approximates between 1/6 and 1/10 of the thickness of said mobile knife.
 10. Apparatus as defined in claim 1, wherein said mobile knife is located in a first plane and one of said counterknives is located in a second plane normally making with said first plane an angle of 1*-6*.
 11. Apparatus as defined in claim 1, wherein one of said counterknives is provided with at least one socket and further comprising a projection mounted in said support and extending into said socket to hold said one counterknife against movement in said direction.
 12. Apparatus as defined in claim 11, further comprising means for adjustably securing said projection to said support.
 13. Apparatus as defined in claim 1, further comprising advancing means for moving sheet material stepwise along a predetermined path extending between said knife and said counterknives, said portion of said mobile knife comprising two spaced apart projections located at the opposite sides of said path and each abutting against said counterknives.
 14. Apparatus as defined in claim 13, wherein said mobile knife further comprises two spaced apart recesses located at the opposite sides of said path and said drive means comprises projections extending into said recesses.
 15. Apparatus as defined in claim 14, wherein said drive means comprises a reciprocable yoke which is provided with said projections.
 16. Apparatus for severing sheet material, particularly for severing paper webs having layers of exposed and developed photosensitive material, comprising a support; two counterknives mounted in said support, said support comprising first and second portiOns which respectively support one and the other of said counterknives and means for separably securing one of said portions to the other portion so as to facilitate removal of the respective counterknife; a mobile knife movable with reference to said counterknives forwardly and rearwardly in a predetermined direction, said counterknives defining a gap for reception of said knife during said forward movement of said knife and said knife having a portion abutting against said counterknives and at least one cutting edge which is inclined relative to said direction; drive means for moving said knife whereby the latter cooperates with said counterknives to sever during the forward movement thereof a sheet material which is placed between said knife and said counterknives; and biasing means for yieldably urging said counterknives against said knife. 